The present invention relates to a communication apparatus of the kind having an optical cable introduced thereinto and more particularly to a structure and a method for the introduction of an optical cable into an outdoor communication apparatus.
While an optical cable is capable of transmitting a greater amount of data than a coaxial cable, its structure lacks in mechanical strength. In a communication network, optical cables are connected to a main line. The low mechanical strength of the cables themselves is particularly problematic at portions where the cables are connected to communication apparatuses, and has critical influence on the entire network. Such connecting portions should therefore be provided with high reliability and high quality.
It is a common practice to introduce an optical cable into a communication apparatus by using any one of four different structures, i.e., a simple clamp structure (first structure hereinafter), a simple water-proof affixing structure using a self-expanding tape (second structure hereinafter), a closure structure (third structure hereinafter), and an outdoor connecting structure (fourth structure hereinafter). These conventional structures have some problems left unsolved, as follows.
The first structure includes claws, which are raised to catch the sheath of an optical cable. The claws are apt to damage the sheath and allow water to enter the apparatus via the sheath. Further, the first structure uses a compressible tubular rubber packing for a waterproofing purpose. This brings about a problem that the tubular rubber packing strongly compressed causes creases to appear between it and the cable or that loads acting on the rubber packing over a long period of time cause the packing to crack. The creases and cracks produce clearances between the cable and the rubber packing, deteriorating the waterproofing ability of the structure. This problem is particularly serious when the cable has a small diameter.
In the second structure, after a self-expanding tape has been wrapped around an optical cable, the cable is inserted into a hole. In this condition, the tape fills a gap between the cable and the edge of the hole due to its own elasticity. However, such a simple waterproof scheme does not work over a long period of time because the tape is deteriorated due to aging. Moreover, the twist or the bend of the cable is directly imparted to the inside of the apparatus and likely to damage optical fibers existing in the apparatus.
The third structure introduces an optical cable into a closure. With this structure, it is necessary to increase the mounting area of an introducing portion within the apparatus.
In the fourth structure, a connector and a clamp are provided in a pair in matching relation to the diameter of an optical cable. This structure therefore needs a great number of different cable introducing portions each matching with a particular cable diameter and lacks in general-purpose applicability.
Generally, optical cables with great outside diameters are used for main lines transmitting a great amount of data while optical cables with small outside diameters are used for terminal lines transmitting a small amount of data. Further, the outside diameter of an optical cable depends on the manufacturer. Therefore, when the cable facilities are moved or varied, the above conventional structures require all the cable facilities to be replaced.
The fourth structure has a problem that when an optical cable is mounted or replaced or when an optical fiber mounted in a connector must have its radius of curvature adjusted, the entire cable introducing portion must be disassembled. This obstructs efficient operation and maintenance. Another problem is that the structure simply affixes a tension member protruding from the cable. In this condition, vibration ascribable to, e.g., winds causes the cable to twist or bend, twisting or pulling or, in the worst case, breaking optical fibers disposed in the connector. A further problem is that the connector cannot be reduced in size because the support of the tension member, the splice of the cable and the bend of the fibers to more than a prescribed radius of curvature (radius R greater than 30 mm) are effected within the connector.
The prerequisites with the introduction of an optical cable into a communication apparatus are as follows. The cable should be held by a force great enough to withstand tensile forces, moments and twists ascribable to, e.g., vibration caused by strong winds or pulling forces, which may accidentally, act on the cable. A sure waterproof implementation must be provided in order to prevent, e.g., rainwater from entering the apparatus via clearances between the apparatus and the cable when the cable is replaced with a cable having a different diameter.
It has also been customary to use a connector consisting of a plug and a receptacle for introducing an optical cable into a communication apparatus. However, this kind of scheme is not reliable from the standpoint of waterproof, tensile strength and electrical characteristic. Further, the connector is expensive. It is therefore desirable to directly introduce an optical cable into a communication apparatus.